The HIV protease enzyme is essential for HIV infection. MK-639 is a specific inhibitor of this enzyme and inhibits HIV in vitro. ZDV in combination with lamivudine has recently been shown to be a potential inhibitor of HIV replication in patients. This study in HIV infected subjects is designed to assess the safety and effect on virus levels of MK-639 given with ZDV and 3TC compared to MK-639 given alone or to ZDV combined with 3TC.